1. Field of the Invention
The invention relates to an engine control device and an engine control method for controlling an operation of a multi-cylinder engine that is capable of Exhaust Gas Recirculation (EGR) and a partial cylinder operation (also referred to as a reduced cylinder operation).
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 60-45767 (JP-A-60-45767), No. 2000-170562 (JP-A-2000-170562), No. 2004-27971 (JP-A-2004-27971), No. 2007-162594 (JP-A-2007-162594), and so on disclose multi-cylinder engines capable of performing a partial cylinder operation. In JP-A-60-45767, for example, an EGR rate (EGRr) (the proportion of recirculated exhaust gas in an intake gas) in a partial cylinder operation is reduced below that of an full cylinder operation as a method of preventing deterioration of the drivability, which is caused by torque variation due to an increase in an combustion interval during the partial cylinder operation and combustion instability due to the effects of EGR.
In this type of engine, a vibration generation condition varies between the full cylinder operation and the partial cylinder operation, and therefore measures must be taken to reduce vibration and noise. During the partial cylinder operation in particular, the ignition/combustion interval among operating cylinders may not be constant (for example, in a virtual V-type four-cylinder operating condition in which two cylinders of a V-type six-cylinder engine are rested, or the like).
In this case, torque variation may occur due to variation in the intake air amount among the operating cylinders (see Japanese Patent Application Publication No. 2007-162606 (JP-A-2007-162606)). Hence, during a partial cylinder operation in which combustion occurs at unequal intervals, vibration and noise may increase. Moreover, as noted above, the implementation of EGR leads to combustion instability, and as a result, a further increase in vibration and noise may occur. Furthermore, variation may occur in the actual EGR rate of the operating cylinders due to variation in the intake condition of the operating cylinders, as described above, and as a result, vibration and noise may increase even further. Therefore, it is particularly important to take measures to reduce vibration and noise during a partial cylinder operation in which combustion occurs at unequal intervals.